IgA, the most abundant antibody synthesized in the body, serves as a first line of immune defense in the mucosal secretions at the interface with the external environment. In our mucosal immunology research, the next phase seeks to provide and extend new insights into the role of IgA in host defense beyond the traditional immunological barrier function in the mucosal secretions. In particular, we want to develop our recent hypotheses for two new defense functions of IgA: (a) to neutralize intracellular microbial pathogens during IgA's transport through the mucosal epithelium and (b) to "excrete" antigens, as immune complexes, from the lamina propria across the epithelial cells into the secretions. The work scope embraces three general topics: the assembly and transport of IgA in and through epithelial cells, IgA as an excretory immunoglobulin, and the disease IgA nephropathy. The first aim is to test directly the hypothesis that final, covalent assembly of secretory IgA during its transport through epithelium is catalyzed by an intracellular enzyme, protein disulfide isomerase, and to identify the subcellular compartment where final assembly occurs. The next aim is to determine exactly which half-cystine residues in the polymeric immunoglobulin receptor (secretory component) are -disulfide bonded to IgA in this assembly. In the studies of IgA as an excretory immunoglobulin we want to investigate the ability of polarized epithelial cell monolayers to transport IgA immune complexes containing polysaccharide as well as protein antigens and to learn whether polysaccharide antigen complexes are handled differently from protein antigen complexes. In this transport system we shall also extend the size range of IgA immune complexes to be excreted to viruses and bacteria coated with IgA. An important part of the effort will be directed to demonstrating that the proposed excretory 1gA immune system operates in vivo. For this we intend, under defined conditions of active and passive immunity, to show the presence in small intestinal crypt cells of antigen-IgA immune complexes in transit from lamina propria to lumen. Finally, we seek to exploit oral enzymes therapeutically in experimental IgA nephropathy, a disease caused by IgA immune complex deposition. Overall, the research aims at understanding better how the mucosal immune system defends against external agents of disease, including infectious, allergenic, and carcinogenic. The insights afforded should aid in developing practical strategies for mucosal immunization.